1. Field of the Invention
The present invention generally relates to particle beam lithography systems and, more particularly, to a particle beam lithography system with in situ calibration and calibration methods therefor.
2. Background Description
Electron beam (e-beam) lithography tools are commonly used in semiconductor manufacturing to form sub-micron shapes on a semiconductor wafer. Shapes are formed by directing a beam of electrons from a source at one end of a column onto a photoresistive layer on a substrate at an opposite end of the column. A typical substrate may be 200-300 mm in diameter or larger. These submicron shapes may be formed either by writing the shape directly onto a photoresistive layer on the substrate, wherein the substrate is a semiconductor wafer; or, by writing the shape onto a photoresistive layer on a substrate which is used as a mask, subsequently, to print the shape onto the semiconductor wafer.
Further, there are two broad types of writing modes used in electron beam lithography. The first type is referred to as xe2x80x9cblind modexe2x80x9d or a xe2x80x9cdead reckoning modexe2x80x9d and is commonly used in mask making. In the blind mode, the substrate is a featureless blank coated with resist and all of the patterns are placed by dead reckoning. The second mode, which may be referred to as the xe2x80x9cregistered write modexe2x80x9d or a xe2x80x9cdirect write mode,xe2x80x9d is commonly used in direct write applications, i.e. writing directly onto a semiconductor wafer, in what are referred to as device fabrication runs. In the registered write mode case, the patterns must be precisely located relative to previous levels which requires registration targets built into each level and the substrate as well. Regardless of the mode employed, accurately placing or repeating sub-micron shapes at precise locations across a distance of 200-300 mm demands precise beam registration.
However, even if the beam is registered adequately when pattern printing begins, during the course of writing the pattern, the e-beam may exhibit what is referred to as drift, i.e., exhibiting increasing inaccuracy in one direction as time passes. So, in order to maintain adequate precision, pattern writing may be interrupted periodically, depending on the particular tool""s inherent e-beam drift, to check tool registration and, whenever registration error exceeds an acceptable tolerance, to adjust the beam.
Normally, the substrate is held on a stage opposite (beneath) the beam source and this registration measurement involves diverting the stage to position a registration target under the beam. Then, the beam is scanned over the registration target, the target""s location is measured and the target""s measured location is compared against an expected result. Any measured errors are corrected by adjusting the beam or adjusting stage positional controls. Then, the stage is returned to its former position to resume writing the mask pattern. This measurement and reregistration can be time consuming.
Furthermore, for this e-beam registration approach, the registration measurement takes place at a stage location other than where the pattern is actually written. Consequently, even after measuring and correcting errors, moving the stage back into position from the measurement area may actually introduce errors, such as from the stage slipping or from other move related stresses. Also, to assure complete accuracy, the beam should be reregistered, frequently, preferably at each field. However, when throughput is a consideration, as it nearly always is, it is impractical to correct the beam registration prior to printing each field.
Consequently, attempts have been made to perform registration in place while writing the pattern, i.e., in situ. One in situ approach, suggested by MIT as reported in xe2x80x9cSpatial-phase-locked Electron-beam Lithography:Inital Test Resultsxe2x80x9d, pp2342-5 J. Vac. Sci. Technol. B. Vol. 11, No 6 November December 1993, is referred to as the Spatial-Phase-Locked E-Beam Lithography (SPLEBL) system. Implementing a SPLEBL type system would require including special registration patterns on every substrate and a blanket exposure of the substrate for the registration cycle. However, such a blanket exposure is unusable for high sensitivity resists. Further, a SPLEBL type system also would require a sophisticated method of extracting positioning information during exposure. While such a sophisticated method may be feasible with a Gaussian or fixed beam shape; it is highly improbable that such a method could be developed for a variable shape beam such as a described in U.S. Pat. No. 4,243,866 entitled xe2x80x9cMethod and Apparatus for Forming a Variable Size Electron Beamxe2x80x9d to Pfeiffer et al.
Another in situ registration technique has been proposed for x-ray membrane exposure by Scientists at Naval Research Laboratories (NRL) as described, for example, by Perkins et al. in xe2x80x9cImproving Pattern Placement Using Through the Membrane Signal Monitoring,xe2x80x9d J. Vac. Sci. Technol. B, Microelectron. Nanometer Struct. (USA) Vol. 16, No. 6 November December 1998 pp3567-71. This technique requires fixing a registration grid to the carrier, directly under the x-ray membrane. The registration grid forms a Schottky diode junction with the carrier, allowing it to function, simultaneously, as a high gain detector of incident electrons. However, this registration technique is limited to high transmission membranes and its resolution capability has not yet been evaluated. Further, below the membrane, the beam diverges and scatters rapidly, which limits the usefulness of this technique.
U.S. Pat. No. 4,119,854 entitled xe2x80x9cElectron Beam Exposure Systemxe2x80x9d to Tanaka et al. teaches an e-beam exposure system that may be compensated for e-beam drift and workpiece drift. The system of Tanaka et al. uses a pair of x and y lines as a reference target. The relative position of the stage with respect to the x and y lines is determined using differential interferometry. A coil is included for refocusing the beam onto the x, y reference target lines. However, refocusing the beam introduces hysteresis into a system such as that taught in Tanaka et al. The hysteresis, itself reduces beam accuracy.
Thus, there is a need for in situ registration methods for e-beam lithography system and more particularly for VAIL e-beam lithography systems.
It is therefore a purpose of the present invention to improve e-beam lithography system registration;
It is another purpose of the present invention to reduce the time required for reregistering e-beam lithography systems;
It is yet another purpose of the present invention to improve VAIL e-beam lithography system accuracy;
It is yet another purpose of the present invention to improve VAIL e-beam lithography accuracy without impacting e-beam tool throughput.
The present invention is an e-beam lithographic system capable of in situ registration. The preferred system is a Variable Axis Immersion Lens (VAIL) e-beam system and is a double hierarchy deflection system. A controllable stage moves a substrate with respect to the beam axis placing the intended substrate writing field within an aperture on a field locking target. The field locking target is located between the optics section and the substrate and the aperture is sized to permit the beam to write the field. The field locking target includes alignment marks around the aperture. A differential interferometric system measures the relative positions of the field locking target and the stage. As the stage is moving into position for writing a field, the beam is swept to hit the alignment marks, checking system alignment. The beam control data (coil currents and electrostatic deflection plate voltages) required to hit the marks are stored, and drift correction values calculated and the field beam control data is compensated. Writing resumes on the newly positioned field with the beam control data corrected by the calculated drift correction values.
The field locking target may include a mechanical adjustment for fine tuning aperture location. After an initial calibration of the system, the field locking target is adjusted to position the alignment marks at their expected location.